Next generation of multifunctional scanning probes

The goal of this thesis was the advanced design, fabrication, and application of
combined atomic force microscopy - scanning electrochemical microscopy (AFMSECM)
probes for high-resolution topographical and electrochemical imaging.
The first part of the thesis describes innovative approaches for the optimization of
AFM-SECM probe fabrication with recessed frame electrodes. For this purpose,
commercial silicon nitride AFM cantilevers were modified using optimized critical
fabrication processes including improved metallization for the deposition of the electrode
layer, and novel insulation strategies for ensuring localized electrochemical signals. As a
novel approach for the insulation of AFM-SECM probes, sandwiched layers of PECVD
SixNy and SiO2, and plasma-deposited PFE films were applied and tested. Using
sandwiched PECVD SixNy and SiO2 layers, AFM-SECM probes providing straight
(unbent) cantilevers along with excellent insulation characteristics facilitating the
functionality of the integrated electrode were reproducibly obtained. Alternatively, PFE
thin films were tested according to their utility for serving as a mechanically flexible
insulating layer for AFM-SECM probes. The electrochemical characterization of PFEinsulated
AFM-SECM probes revealed excellent insulating properties at an insulation
thickness of only approx. 400 nm. Finally, AFM-SECM cantilevers prepared via both
insulation strategies were successfully tested during AFM-SECM imaging experiments.
In the second part of this thesis, disk-shaped nanoelectrodes were for the first time
integrated into AFM probes for enabling high-resolution AFM-SECM measurements.
Disk electrodes with an electrode radius < 100 nm were realized, which provides a
significantly improved lateral resolution for SECM experiments performed in
synchronicity with AFM imaging. Furthermore, the developed fabrication scheme
enables producing AFM-SECM probes with integrated disk nanoelectrodes at
significantly reduced time and cost based on a highly reproducible semi-batch fabrication
process providing bifunctional probes at a wafer scale. The development of a detailed
processing strategy was accompanied by extensive simulation results for developing a
fundamental understanding on the electrochemical properties of AFM-SECM probes with
nanoscale electrodes, and for optimizing the associated processing parameters. Thus
fabricated probes were electrochemically characterized, and their performance was
demonstrated via bifunctional imaging at model samples.
The third part of this thesis describes the development and characterization of the
first AFM tip-integrated potentiometric sensors based on solid-state electrodes with submicrometer
dimensions enabling laterally resolved pH imaging. Antimony and iridium
oxides were applied as the pH sensitive electrode material, and have been integrated into
the AFM probes via conventional microfabrication strategies. The pH response of such
AFM tip-integrated integrated pH microsensors was tested for both material systems, and
first studies were performed demonstrating localized pH measurements at a model system.